There have long been a number of problems encountered in achieving optimum flow patterns for small quantities of liquids being conducted through flow paths associated with analytical apparatus such as, for example, optically-sensitive apparatus like refractometers, ultraviolet absorbtion apparatus, etc. Such problems are also encountered in very small valves which are used with liquid analytical apparatus, e.g. liquid chromatography systems.
In general all of these problems relate to obtaining dependable displacement of the small quantity of liquid-- often injected into a liquid carrier in microliter or submicroliter quantities-- without undue dilution caused by excessively turbulent flow patterns and without undue contamination resulting from a previously utilized liquid or from leaching elastomeric seals. And, of course, the suitability of the apparatus for use at high pressures without leaking is generally a prerequisite for such flow paths.
Attempts to provide valves having desired reliability have included the use of relatively rigid plastic materials. (By "relatively rigid" is meant rigid as opposed to elastomeric synthetic polymers of the type used in gaskets at normal pressures.) These rigid materials have been used in making seats or other seal structure for valves used in injector mechanisms. Problems arise in use of such materials, they tend to absorb (and desorb) some of the broad spectrum of chemical compounds, with which they can be expected to contact. This feature not only tends to change their dimensions but also tends to provide a source for contaminants. Some tend to cold flow at high pressures. This becomes a major problem, even with such relatively creep-resistant materials as the polyacetal-type, whenever an attempt is made to incorporate them in tiny valves meant to moderate the flow of batch samples in the microliter range.
The thrust of innovation by others in this area appears to be development of small volume valves that really serve as sample holding devices e.g. slide valve as described in U.S. Pat. No. 3,748,833 to Karas et al, and can be pivoted or pushed from a sample-receiving position to a sample-flushing position. These devices are not particularly advantageous. Many depend upon elastomers for sealing limits; such dependence interferes with the structural stability of the valves because of cold flow and wear.